1. Field of the Invention
This invention relates to a process for resin desmearing and etchback, especially in the manufacture of printed wiring boards.
2. Brief Description of the Prior Art
Reliability in a multilayer circuit or wiring board is highly dependent on the fabricator's ability to achieve sound mechanical and electrical connections to internal circuits and ground planes. One of the problems that can threaten reliability, known as "drill-smear"--a dielectric material deposited on the hole walls of a printed wiring board during the drilling operation. This material must be removed if reliable interconnections are to be achieved.
C. F. Coombs, Jr. in "Printed Circuits Handbook", third edition, 1988, McGraw-Hill Book Co., section 29.6.13 defines etchback as a process for the controlled removal of nonmetallic materials from sidewalls of holes to a specified depth. It is used to remove resin smear and to expose additional internal conductor surface.
D. R. Witherell in Smear Removal Methods, the Multilayer Printed Circuit Board Handbook, Electrochemical Publications Ltd., 1985, Chapter 11, states that two items are often used when referring to the smear removal processes. The first is known as "chemical cleaning", a process designed for the removal of smear from interconnect surfaces only. No attempt is made to remove dielectric material from the hole walls between individual layers of circuitry although a small amount of removal is a natural consequence of the smear removal process.
A chemical process, designed to remove the smear as well as a layer of nonmetallic materials such as glass/epoxy between the interconnects, is known as "etchback". The depth of etchback can be varied from something less than 0.0025 mm to a depth as great as 0.075 mm, depending on customer preference. With glass/epoxy laminates, concentrated sulfuric acid is a selective etchant leaving the glass strands exposed in the hole walls. These strands must be removed in a secondary operation to prevent the growth of plating nodules in subsequent plating steps. This additional operation is not normally required with the chemical cleaning process.
Hydrofluoric acid or ammonium bifluoride are often used to remove the glass fibers in a separate step following etchback. In some applications, the hydrofluoric acid or ammonium bifluoride are mixed with the sulfuric so that the etchback and glass fibre removal can be accomplished in a single step.
U.S. Pat. No. 4,601,784 (Krulik) describes aqueous alkaline liquid NaMnO.sub.4 solutions for resin desmearing and etchback in the manufacture of printed circuit boards. The solutions comprise water, alkali metal hydroxide, NaMnO.sub.4, and from about 0.1 to about 3.0 moles per mole of MnO.sub.4 - of a co-ion for MnO.sub.4 - selected from K.sup.+, Co.sup.+, Rb.sup.+, and mixtures thereof.
Numerous U.S. and foreign patents and published foreign patent applications have issued which teach different permanganate desmearing and neutralization compositions and/or desmearing or neutralization operations. U.S. Pat. No. 3,962,496 (Leech) teaches of a hydrazine neutralizer solution containing a sequestering agent (e.g. ethylenediamine tetraacetic acid, sodium tartrate, and triethanolamine) and a pH adjustor (e.g. sodium hydroxide, potassium hydroxide, and sodium carbonate).
U.S. Pat. Nos. 4,042,729 (Polichette, et al.) and 4,073,740 (Polichette, et al.) teach a composition for treating a resinous surface to later receive a deposit of electrolessly-formed metal, said composition comprising water, permanganate ion and manganate ion, wherein the molar ratio of manganate ion to permanganate ion is up to 1.2 to 1 and said composition having a pH in the range of 11 to 13.
U.S. Pat. No. 4,054,693 (Leech, et al.) teaches a process of treating a resinous surface by first contacting that surface with the same permanganate ion/manganate ion solution as used in the preceding two patents, then neutralizing the treated resin surface with an aqueous solution comprising hydrazine and then following that neutralization with metallizing that resinous surface with an electroless metal deposition bath.
U.S. Pat. No. 4,233,344 (Braach) teaches treating a composite substrate with a copper-type colloidial system to cause activation of the nonconductive portions thereof for electroless metal deposition, and thereafter treating the activated substrate with an adhesion promoter (i.e., hydrazine hydrate, ammonium persulfate, or alkali hydroxide) prior to electroless metal deposition.
U.S. Pat. No. 4,425,380 (Nuzzi, et al.) teaches a process for preparing a resinous substrate for subsequent metallization, said process comprising first contacting the substrate with an alkaline permanganate treating solution, then contacting said substrate with a water-soluble compound (e.g., tin chloride, sodium bisulfite, hydrochloric acid, or hydroxylamine hydrochloride) to reduce any manganese residues deposited on said substrate to a low oxidation state, and finally contacting said substrate with an alkaline hydroxide solution to remove essentially all of said manganese residues.
U.S. Pat. No. 4,430,154 (Stahl, et al.) teaches a process for making printed circuit boards involving the steps of removing an adhesive coating by treating the board with an aqueous solution containing potassium permanganate and sodium hydroxide, and thereafter treated with an aqueous solution of hydrochloric acid or hydrazine hydrate.
U.S. Pat. No. 4,515,829 (Deckert, et al.) teaches an overall process for manufacturing a printed circuit board having a plurality of metal plated holes interconnecting at least two circuits, including the steps of drilling holes in an epoxy board, forming the circuits, contacting the hole walls with an aqueous alkaline oxygenated epoxy solvent at a pH greater than 10, then contacting the holes with an aqueous alkaline permanganate solution at an elevated temperature and a pH in excess of 13, and also contacting the hole walls with a reducing agent solution.
U.S. Pat. No. 4,592,852 (Courduvelis, et al.) teaches an alkaline composition to improve the adhesion of plastics to electroless metal deposits, said composition containing a source of permanganate ions and a secondary oxidant selected from the group consisting of chlorine, bromine, ozone, hypochlorite salts, metaperiodate salts and trichloro-s-triazinetrione salts.
U.S. Pat. No. 4,592,929 (Tubergen, et al.) teaches a process for the metallization of a plastic which includes the steps of first treating the plastic with a liquid permanganate oxidant solution, then contacting the plastic with a solution containing a reducing agent, a pH adjustor to render the reducing agent active, and a surface active agent in sufficient concentration to reduce the surface tension of the solution to 50 dynes per centimeter or less.
U.S. Pat. No. 4,629,636 (Courduvelis, et al.) teaches a process to improve the adhesion of a plastic to an electroless metal deposit wherein said plastic is contacted with an alkaline permanganate solution which contains permanganate ions, manganate ions, and a secondary oxidant; the secondary oxidant being added at controlled intervals to keep the ratio of permanganate ion concentration to the sum of permanganate and manganate ion concentrations above about 0.5.
U.S. Pat. No. 4,698,124 (Krulik) teaches a method for regenerating spent permanganate ions in a permanganate-containing etchant composition comprising periodically adding an oxidizer selected from the group consisting of an inorganic peroxy disulfate, mixtures of an inorganic peroxy disulfate, and an inorganic hypochlorite, and mixtures of an organic peroxy disulfate, and an inorganic chlorate in an amount to oxidize essentially all of the nonpermanganate manganese species in the composition to Permanganate.
Japanese Pat. No. 81-003373 (which issued on Jan. 24, 1981) and Japanese Pat. No. 81-015736 (which issued on Apr. 11, 1981) teach the use of alkaline solutions of potassium permanganate and sodium or potassium hypochlorite in the treatment of ABS resins prior to electroless metal plating. The alkaline solutions include those having a pH in the range of 11.0 to 12.35 and 12.0 to 13.5, respectively.
Japanese Kokai No. 79-055933 and Japanese Kokai 79-117,328, the latter published on Sept. 12, 1979, teach an electroless plating on plastics process involving etching the plastics with aqueous solution containing potassium permanganate and persulfate prior to electroless metal plating.
U.S. Pat. Nos. 4,619,741; 4,684,560 and 4,724,005, which issued to Karl L. Minten and Galina Pismennaya, on Oct. 28, 1986; Aug. 4, 1987; and Feb. 9, 1988, respectively, teach a process of electroplating the through holes of a PWB which is a significant improvement over the known electroless techniques. By this process, a liquid dispersion of carbon black particles is first applied to the nonconductive portions of the through holes; then the liquid dispersion medium is separated (i.e., evaporated) from the carbon black particles, thereby depositing a substantially continuous layer of carbon black particles on the nonconductive surfaces of the through holes; and next a substantially continuous metal layer is electroplated over the deposited carbon black layer. This process of Minten and Pismennaya has several advantages over the known electroless techniques including the elimination of the preactivator, the Pd/Sn activator and the accelerator; less possibility of pollution problems; better bath stability; and fewer possible side reactions. This disclosure of the above-mentioned U.S. Patents of Minten and Pismennaya is incorporated herein by reference in their entirety.
Improvements and modifications of this Minten and Pismennaya process are shown in U.S. Pat. Nos. 4,622,107 (Piano); 4,622,108 (Polakovic and Piano); 4,631,117 (Minten, Battisti, and Pismennaya); and 4,718,993 (Cupta and Piano); 4,874,477 (Pendleton) and U.S. Pat. No. 4,897,164 (Piano and Galvez). The first of these patents teaches the use of a gas-forming compound (e.g. sodium carbonate) to remove loose or easily removable carbon black particles in the through holes. The second of these patents teaches the contacting of a alkaline hydroxide preconditioning solution to the through hole walls before application of the carbon black dispersion so that the carbon black dispersion will have better adhesion to the walls. The third listed patent teaches the use of this carbon black dispersion as a preactivator for electroless plating of the through holes. The fourth teaches the use of an alkaline silicate solution before the carbon black dispersion. The fifth patent teaches the use of an aqueous polyelectrolyte homopolymer conditioner solution before the carbon black dispersion bath. The sixth patent teaches the use of an alkaline borate solution to remove excess carbon black material on the rims and inner metal walls of the PWB through hole walls which might cause an undesirable, uneven plated surface to result. All of the above U.S. and foreign patents and publications are incorporated herein by reference in their entireties.